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S. Goldammer et al. /New fluoride-conducting glass-ceramics 381

an iron mould and quenched on air, and annealedwith 5 K/min starting at T,. he glass forming re-gion is shown in fig. 1. Glasses within this region weretransparent and free of crystals, bubbles and streaks(circles in fig. 1). Glass transformation temperatureT, nd the onset temperature of crystallization T,were measured by DTA-analysis at a heating rate of5 K/min. Heat treatment at temperatures betweenTg+ 0 K and T, 20 K and a duration from 30 minto 45 h was applied to form glass-ceramics.

Conductivity measurements were carried out onpolished samples of 2 mm thickness. Aluminiumelectrodes were evaporated on both sides of the sam-ples in a guard ring arrangement. Measurements wereperformed with an impedance analyser SI 1260 fromSchlumberger in the frequency range from 1 Hz to1 MHz at temperatures ranging from room temper-ature to T,. C conductivity was determined fromthe complex impedance plot as usual.Crystalline phases were determined by means ofX-ray diffraction. In some cases electron microscopywas used to get information about the morphologyof the glass-ceramics, i.e. arrangement, size and ori-entation of crystalline phases.

3. ResultsThe conductivity of all glasses is thermally acti-

vated showing an Arrhenius type dependence ontemperature. With increasing F- concentration con-ductivity is strongly increased (fig. 2a) while the ac-tivation energy is lowered (fig. 2b). The substitutionof SiOZ by PbO has only a small influence on con-

PbF>

-14,5 , / I I I I0 2 4 6 8 10 12

C, (lO"/cm')194

Mel% SiO,0 50 A40 X 33,33 0 30 0 25 l 20

192

0 2 4 6 8 10 12 14c, (10"/cm3)

Fig. 2. Conductivity of glasses in the system Si02-PbO-PbF2 at100C as function of fluoride concentration; (b) activation en-ergy of conductivity as function of fluoride concentration.

R-4

AL L3b3Si207

Pb2SnOLFig. 1. Glass-forming region in the system SiO*-PbO-PbF,.


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382 S. Goldammer et al. /New fluoride-conducting glass-ceramics

ductivity. The dependence of conductivity on the ra-tio of SiOZ to PbO vanishes at higher fluorideconcentration.

Depending on glass composition and heat treat-ment crystallization of the glasses has a different in-fluence on conductivity. Systematic variations oftemperature and duration of the thermal treatmentrevealed two distinct types of crystallization behav-ior (type I and type II).

Type I: In glasses with up to 15 mol% PbFz con-ductivity is increased with the onset of crystalliza-tion at about 20 K above TK Maximum conductivitychanges observed so far for this type of glasses aresummarized in fig. 3. Two crystalline phases Pb&O,and PbF, have been established in the glass-ce-ramics. For type I glasses heat treatment slightlyabove 7, leads to the formation of crystallinePb&O,. The appearance of this crystalline phase isaccompanied with a strong increase of conductivity(fig. 4). At higher temperature, a second crystallinephase PbFz is formed. At the same time conductivitydecreases. Thus, conductivity passes through a max-imum value.

Type II: Glasses with more than 15 mol% PbFz be-long to the second type of crystallization behavior.In contrast to type I, heat treatment at 20 K aboveTB leads to the formation of PbFz crystals. At the sametime the conductivity decreases (fig. 5). This is ac-companied with an increase of activation energy (fig.6, glass with 33.3 mol% SiO, after 45 h at 300C).

- 1 0 0 0 / l CK- ' 1 -Fig. 3. Conductivity of glasses and glass-ceramics with up to 15mol% SO2 (type 1)

-63 ( - I'hl;z1 I%$3207

- 1 (;hlss - - Glass-ceramic-7 D /300 350

temperature [ClFig. 4. Type 1: influence of heat treatment on crystalline phasesand conductivity.

-5

-6

-7S::i;2 -8

300 350temperature [Cl

400

Fig. 5. Type II: influence of heat treatment on crystalline phasesand conductivity.

Further heat treatment at higher temperatures yieldsa crystallization of the second phase Pb&O, (fig.5). Now, conductivity rises again while the activa-


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S. Goldam mer et al. /New fluoride-conduct ing glass-ceramics 383

-5 - _ Glass- _ _ Gl a s s - c e r a mi c

-s -

2.2 2,L 2.6 2,s 30 3.2- 1 0 0 0 / T t K- ' 1 -

Fig. 6. Conductivity of glasses and glass-ceramics with more than15 mol% SiOl (type II).tion energy is lowered, in some cases reaching values(0.25 eV) smaller than for pure PbF2 (0.32 eV).The glass-ceramics were investigated by means ofscanning electron microscopy. Glass-ceramics of typeI showed spherical agglomerations of Pb&O, (5-10 pm) distributed within the remaining glass ma-trix. PbF2 crystals arising in type II and-at highertemperature-in type I glass-ceramics are too smallto be resolved by scanning electron microscopy ( < 10nm ). Further investigations using replica techniquesare in progress to get more detailed information aboutthe morphology of the glass-ceramics at the initialstate of crystallization.

4. DiscussionFluoride conduction in the glasses of the system

SiOz-PbO-PbF2 shows the same dependence onstructure and mobile ion concentration as alkali con-duction in oxide glasses. This similarity is the resultof the action of PbF2 as a network modifier in thesame way as NazO in silicate glasses. This is possiblebecause in glasses with high lead oxide content thePbO behaves as a network-former giving rise tobridging oxygen (Pb-0-Pb) [ 121. When PbO issubstituted by PbFZ bridging oxygen is removed whilefluoride ions are bound to Pb2+. As a consequenceglass transformation temperature is lowered andconductivity increases exponentially with fluorideconcentration up to a saturation at high PbF2 content.

The conversion of a glass to a glass-ceramic leadsto a mixture of crystalline and glassy phases. Theconductivity of such heterogeneous system dependsnot only on the volume fractions and conductivitiesof the different compounds. It is also strongly influ-enced by transport processes along grain boundariesand by the morphology of the system, i.e. shape, sizeand orientation of the compounds and its spatial ar-rangement. Actually, further research is in progressto investigate these details in order to get necessaryinformation for model calculations.

Compositions and morphology of the glass-ce-ramics are not identical with the equilibrium state ofthe systems at the given temperatures of heat treat-ment. As the heat treatment was interrupted beforethe equilibrium was reached, a transition state of thesystem on its way to equilibrium was frozen in. Longtime heat treatment leads always to the formation ofboth PbF2 and Pb3Si207, in some cases a smallamount of an unidentified third crystalline phase wasobserved. However, at short times and at tempera-tures slightly above T, only a single crystallinephase-PbF, or PbJSi20+s obtained depending onthe PbF2 content in the glass. Obviously at high PbF2content only slight rearrangement of the glass struc-ture is required to form PbF2 (type II, > 15 mol%PbF2). On the other hand at low PbF2 content (typeI, I15 mol%) the formation of Pb$i207 is pre-ferred. In both cases this single crystalline phase arisesas isolated particles which are enclosed in a glass ma-trix. Thus, the total conductivity is mainly deter-mined by the remaining glass matrix. The conduc-tivity of this glass depends strongly on fluorideconcentration while the network former componentshave only a small influence (fig. 2).

When Pb3Si20, is formed the F- concentration inthe remaining glass increases. That is why the con-ductivity of type-1 glasses increases with the onset ofcrystallization. In contrast, the glass matrix loses F-when PbF2 is crystallized. Consequently conductiv-ity of the glass matrix decreases. This is the case whenheat treatment of type-1 glasses is continued for alonger time or at higher temperature.In the high fluoride glasses (type II) the conduc-tivity decreases with the onset of crystallization be-cause the PbF2 crystals are isolated from each other.However, when heat treatment is continued at highertemperature and Pb$i20, is crystallized, too, the


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384 S. Goldammer et al. /NewJluoride-conducting glass-ceramics

PbFz begins to build a continuous network. Then thetotal conductivity is mainly determined by the con-ductivity of this network. Interesting to note is thatin some cases the activation energy of conductivitywas found to be lower than for pure PbF2. Possiblythis phenomenon can be explained by increasingmobility of fluoride ions along the grain boundariesof lead fluoride crystals. Such effects have been ob-served in mixtures of ionic conductors with fine dis-persed insulating materials [ 13 1.

The present results confirm that it is possible tochange the conductivity of fluoride conducting glassesin the system SiOz-PbO-PbF2 in a defined way bymeans of controlled crystallization. Both increase ordecrease of conductivity can be achieved up to a fac-tor of three decades. In a further study quantitativerelations between the content, size and spatial dis-tribution of crystalline phases and the conductivityof the glass-ceramics will be established.

References

[ 1 ] H.L. Tuller, D.P. Button and D.R. Uhlmann, J. Non-Cryst.Solids 40 ( 1980) 93.

[2] D. Leroy and D. Ravaine, C.R. Acad. Sci. (Paris) 287C(1987) 455.[31 G.V. Chandrashekhar and M.W. Shafer, Mat. Res. Bull. I5(1980) 221.

[4] P.C. Schultz and MS. Mizzoni, J. Am. Ceram. Sot. 56(1973) 65.[51 J.E. Shelby, J. Am. Ceram. Sot. 68 ( 1985) 55 1.

[6] J.Co0nandJ.E. Shelby,J. Am.Ceram. Sot. 71 (1988) 354.[7] A. Osaka, Y. Wang, M. Kobayashi, Y. Miura and K.

Takahashi, J. Non-Cryst. Solids 105 ( 1988) 63.[81 Y. Wang, M. Kobayashi, .4. Osaka, Y. Miura and K.Takahashi, J. Am. Ceram. Sot. 71 (1988) 864.

[9] Y. Wang, A. Osaka and Y. Miura, J. Non-Cryst. Solids 112(1989) 323.[ lo] H. Kahnt and J.M. Reau, Phys. Status Solidi (a) I21 ( 1990)73.[ 111 Ch. Kaps and H. Kahnt, private communication.

[ 121 W. Vogel, Glaschemie (VEB Deut. Verlag furGrundstoffindustrie, 1975).[ 131 A.K. Shukla, R. Manoharan and J.B. Goodenough. SolidState Ionics 26 ( 1988) 5.

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